Some conventional HVAC systems are capable of employing dry contact (discrete on-off switch) indoor air quality sensors to detect when carbon dioxide (CO2) or volatile organic compounds (VOCs) reach a level of concentration regarded as potentially harmful. The design of the sensors themselves reflects that level of concentration. For example, one dry contact CO2 sensor may be designed to change state when the CO concentration reaches 1000 parts per million (PPM), while another may be designed to change state when the CO2 concentration reaches 5000 PPM. Conventional HVAC systems have employed dry contact indoor air quality sensors to advantage, and customers and users of such systems have benefited from the integration of such sensors into such systems.